Saudi journalist Jamal Khashoggi speaks on his cellphone at the World Economic Forum in Davos, Switzerland in 2011. AP

Saudi Attorney General Sheikh Saud Al Mojeb said on Saturday that the writer and critic Jamal Khashoggi died after talks at the consulate degenerated into a fight.

Eighteen days after the mysterious disappearance of Khashoggi, Saudi Arabia admitted the 59-year-old was killed inside the kingdom's Istanbul consulate. The admission came after claims by the Saudi authorities that the journalist had left the consulate alive.

Eighteen suspects have been detained in relation to the case. Deputy intelligence chief Ahmad Al Assiri and royal court media adviser Saud Al Qahtani, both part of Crown Prince Mohammed bin Salman's inner circle, were sacked.

"Discussions that took place between him and the persons who met him … at the Saudi consulate in Istanbul led to a brawl and a fist fight with the citizen, Jamal Khashoggi, which led to his death," the attorney general said in a statement.

An official Saudi source said the Kingdom “expresses its deep regret at the painful developments that have taken place and stresses the commitment of the authorities in the Kingdom to bring the facts to the public”.

The kingdom's investigation had so far shown that the suspected killer travelled to Istanbul to meet Khashoggi to discuss the possibility of the writer's return to Saudi Arabia.

The victim had been living in exile.

"The discussions that took place with the citizen … during his presence in the consulate did not go as required and developed in a negative way," the anonymous Saudi official said in a statement.

The saga has put the kingdom and its young crown prince under pressure to offer a feasible explanation for the death of Khashoggi. Saudi officials have roundly denied that Prince Mohammed had any involvement.

The UAE maintains its support for the Saudi handling of the investigation into the death of Mr Khashoggi, which state media WAM described as an “unfortunate and tragic incident”.

Sheikh Abdullah bin Zayed Al Nahyan, Minister of Foreign Affairs and International Cooperation, commended King Salman for “his great efforts to explore the truth and seek legal accountability,” which he said “reflects the transparency and justice in his decision-making on the case.”

The governments of Egypt and Yemen also expressed support for the decisions made by the Saudi king concerning the death of Khashoggi

President Donald Trump endorsed Saudi Arabia's story, terming it an "important first step" and telling reporters on Friday that he found it to be credible. Other leaders have expressed doubt about the new version of events.

German Chancellor Angela Merkel said she didn't accept Saudi Arabia’s explanation. The “horrific events” surrounding Khashoggi's death "still haven’t been cleared up and of course we demand that they be cleared up," she told a regional convention of her Christian Democratic Union party on Saturday.

Danish Prime Minister Lars Lokke Rasmussen also said he was unconvinced.

“The fact that the Saudis last night confirmed that he died, after previously insisting he left the consulate alive, shows that we haven’t been told the full truth, and we must insist on getting that,” Mr Rasmussen said.

US legislators, Republican and Democrat, have expressed their doubts.

UN chief Antonio Gutterres said he was "deeply troubled" by the kingdom's disclosure on Saturday, adding there needed to be "full accountability for those responsible".

Senior scholars in Saudi meanwhile have praised the kingdom's response. The country's highest religious body, the Council of Senior Scholars, on Saturday said the king's decisions on the death of Khashoggi would "achieve justice and equality in accordance with Islamic law", according to a statement on state news agency SPA.

Walid Al-Sama'ani, Saudi's justice minister, said on Saturday that the judiciary had "full independence" to deal with the Khashoggi case and that "the issue will take its regular course...and will reach the judiciary after the completion of the prosecution requirements", according to the Saudi Press Agency.

The minister warned that the judicial process would not be destabilised by any "aggressive behavior, practiced by others via reckless means of media and lack of professionalism and credibility" and that accountability in this matter would be proven.

Saudi Arabia's admission comes after Turkish authorities widened their investigation on Friday, searching a forest in Istanbul where the body might have been dumped, and interviewing the consulate's staff.